chainHead: Produce method responses on `chainHead_follow`

[lexnv]

This PR extends the chainHead_follow to produce the results of the other chainHead methods

• chainHead_storage, chainHead_call, chainHead_body are now plain methods
• mpsc::channel is added between the chainHead_follow and the rest of the methods
• unique operation ID to identify the response in the chainHead_follow events is generated by the subscription management
• BlockGuard struct is extended with the following fields:
  • the unique operation ID
  • the Sender part of the channel to communicate the response back to the chainHead_follow
• chainHead_follow merges the Recv part of the channel to forward responses of methods back to the user
• remove the events produced by the prior subscriptions that are now plain methods
• testing is adjusted and added unique ID validation

Closes: https://github.com/paritytech/substrate/issues/14638

// @paritytech/subxt-team

[lexnv]

bot rebase

[paritytech-processbot[bot]]

Rebased

[lexnv]

bot rebase

[paritytech-processbot[bot]]

Rebased